bitfield-internal.h: fix negative value shifting warnings

bitfield-internal.h: fix negative value shifting warnings

This patch makes _bt_unsigned_cast() shift the complement of 0 casted to
uint64_t instead of the complement of 0 casted to a possibly signed
type, which results in shifting a negative value, an undefined operation
in C.

Because we're imposing uint64_t now, the size of the type and value's
type cannot be greater than the size of uint64_t... not a considerable
worry with the current state of commercial computing. Having said that,
the new _bt_check_max_64bit() macro, a static assertion, checks that
those type sizes are not greater than the size of uint64_t at build
time.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>